1. Field of the Invention
The instant invention is directed to evaluating the strength of adhesion bonds. In particular, non-destructive bond strength tests are accomplished on the bonded articles using Stress Wave Emission techniques.
2. Description of the Prior Art
The instant invention will be described in relation to the non-destructive testing of beam lead devices. However, it will be appreciated that the instant method can be advantageously used to test various types of connections such as crossovers, lead frame bonds or the like.
Beam lead devices have a central body or chip portion with a plurality of laterally extending beam leads. These leads are thermocompressively bonded to metallic bonding pads on insulative substrates having conductive patterns thereon. Such a bonding technique results in the leads being firmly attached to the metallic pads while the chip portion thereof is spaced from the substrate.
Once bonded, the strength of such adhesion must be determined prior to further assembly. A number of tests have been devised in order to make such a strength determination. One such test provides a hook or a clamp to grasp the chip and apply a predetermined upward force to the device. Additionally, an upward force may be applied with an air blaster having at least a pair of opposed air ducts which simultaneously direct air beneath the chip portion as described in U.S. Pat. No. 3,759,088 which issued on September 18, 1973 to N. E. Hardwick III. The opposed impinging air jets induce a predetermined resultant force on the underside of the chip portion causing poorly bonded devices to be torn away from the substrate. However, with these testing techniques, the results are conclusive only if most of the leads are unsoundly bonded and the device torn away from the substrate. Such a test cannot readily identify situations in which only one or two of the perhaps sixteen beam leads are poorly bonded. If only a few leads are unsoundly bonded, the device would not be torn away from the substrate, for the sound bonds would tend to hold the device in place.
Another technique commonly used is to apply a continuous peel force to a small percentage of devices in a particular lot. The applied force continues until the devices peel away from the substrate and such force is recorded. If less than a predetermined small percentage of devices fail below an acceptable peel force value, the entire lot is regarded as acceptable; if the percentage of devices failing is unacceptable, the entire lot is discarded. Such destructive testing is not entirely satisfactory due to the expense associated with the loss of product which must be destroyed during testing and the inherent uncertainty which is associated with such statistical testing.
Another testing technique described in U.S. Pat. No. 3,559,054 which issued to R. W. Bowers on Jan. 26, 1971, directs a high velocity air blast at a device while simultaneously monitoring changes in electrical resistance of the bonded joint connecting the device to the substrate. Such a technique requires a substantially continuous blast of air which can blow the device from the substrate.
A further method and apparatus for non-destructively evaluating the strength of a bond in real time is set forth in U.S. Pat. No. 4,004,456 which issued on Jan. 25, 1977 to S. J. Vahaviolos and is assigned to the instant assignee. This patent teaches the measuring of stress wave emissions emanating from the bonding site during the compression phase of the formation of an adhesion bond and compares this measurement with a predetermined substantially linear relationship between the emitted stress wave energy and the strength of the bond. Although such a method is quite effective for determining the bond strength during the formation of a bond, it is inapplicable for testing bond strengths after the bond has been made.
Accordingly, there is a need for a fast, accurate non-destructive test of bond strength subsequent to adhesive bonding.